Doc: Makes requested changes to wording.

Resolves: #14180

Author: pdpark

doc/source/groupby.rst

@@ -944,15 +944,15 @@ that is itself a series, and possibly upcast the result to a DataFrame:
    So depending on the path taken, and exactly what you are grouping. Thus the grouped columns(s) may be included in
    the output as well as set the indices.
 
-.. warnings::
+.. warning::
 
     * In the current implementation apply calls func twice on the
     first group to decide whether it can take a fast or slow code
     path. This can lead to unexpected behavior if func has
     side-effects, as they will take effect twice for the first
     group.
 
-    * Apply should not perform in-place operations on the group chunk. 
+    * Apply should not apply functions to the group chunk. 
     Group chunks should be treated as immutable, and changes to a 
     group chunk may produce unexpected results.